The present invention relates to an interferometer system for measuring the shape of an aspheric surface of an optical element in an optical system and for measuring the wavefront aberration of such an optical system, particularly in connection with manufacture of a projection optical system suited to for use in an exposure apparatus employing soft-X-ray (EUV) exposure light.
Light of wavelength 193 nm or longer has hitherto been used as the exposure light in lithographic equipment used when manufacturing semiconductor devices such as integrated circuits, liquid crystal displays, and thin film magnetic heads. The surfaces of lenses used in projection optical systems of such lithographic equipment are normally spherical, and the accuracy in the lens shape is 1 to 2 nm RMS (root mean square).
With the advance in microminiaturization of the patterns on semiconductor devices in recent years, there has been a demand for exposure apparatus that use wavelengths shorter than those used heretofore to achieve even greater microminiaturization. In particular, there has been a demand for the development and manufacture of projection exposure apparatus that use soft X-rays of wavelength of 11 to 13 nm.
Lenses (i.e., dioptric optical elements) cannot be used in the EUV wavelength region due to absorption, so catoptric projection optical systems (i.e., systems comprising only reflective surfaces) are employed. In addition, since a reflectance of only about 70% can be expected from reflective surfaces in the soft X-ray wavelength region, only three to six reflective surfaces can be used in a practical projection optical system.
Accordingly, to make an EUV projection optical system aberration-free with just a few reflective surfaces, all reflective surfaces are made aspheric. Furthermore, in the case of a projection optical system having four reflective surfaces, a reflective surface shape accuracy of 0.23 nm RMS is required. One method of forming an aspheric surface shape with this accuracy is to measure the actual surface shape using an interferometer and to use a corrective grinding machine to grind the surface to the desired shape.
In a conventional surface-shape-measuring interferometer, measurement repeatability is accurate to 0.3 nm RMS, the absolute accuracy for a spherical surface is 1 nm RMS, and the absolute accuracy of an aspheric surface is approximately 10 nm RMS. Therefore, the required accuracy cannot possibly be satisfied. As a result, a projection optical system designed to have a desired performance cannot be manufactured.
So-called null interferometric measurement using a null (compensating) element has hitherto been conducted for the measurement of aspheric surface shapes. Null lenses that use spherical lenses comprising spherical surfaces, and zone plates wherein annular diffraction gratings are formed on plane plates have principally been used as null elements.
FIG. 1 shows a conventional interferometer system 122 arrangement for null measurement using a null (compensating) element 132. The interferometric measurement described herein is a slightly modified version of a Fizeau interferometric measurement. Namely, a plane wave 126 emitted from an interferometric light source 124 is partially reflected by a high-precision Fizeau surface 130 formed on a Fizeau plane plate 128. The component of plane wave 126 transmitted through Fizeau surface 130 is converted into measurement wavefront (null wavefront) 134 by null element 132 and assumes a desired aspheric design shape at a measurement reference position RP, following which it arrives at a test surface 138 of a test object 136 previously set at the reference position. The light arriving at test surface 138 is reflected therefrom and interferes with the light component reflected from Fizeau surface 130, and forms monochromatic interference fringes inside interferometer system 122. These interference fringes are detected by a detector such as a CCD (not shown). A signal outputted by the detector is analyzed by an information processing system (not shown) that processes the interferometer information contained in the output signal. Similar measurements can be performed using a Twyman-Green interferometer.
To accurately ascertain the shape of test surface 138, the null element 132 must be manufactured with advanced technology, since there must be no error in the null wavefront. Specifically, this means that the optical characteristics of the null element 132 must be measured beforehand with high precision. Based on these measurements, the shape of null wavefront 134 is then determined by ray tracing. This results in the manufacture of null element 132 taking a long time. Consequently, the measurement of the desired aspheric surface takes a long time.
FIG. 2 shows another example of a conventional Fizeau interferometer 222. Referring to FIG. 2, laser light from laser 224 passes through a lens system 226 to become a collimated light beam of a prescribed diameter and is incident Fizeau plate 228. Rear side 230 of Fizeau plate 228 is accurately ground to a highly flat surface, and the component of the incident light reflected by rear side 230 of Fizeau plate 228 becomes a reference beam having a plane wavefront. The component of incident light transmitted through a Fizeau plate 228 passes through null element 232, where the plane wavefront is converted to a desired aspheric wavefront. The aspheric wavefront is then incident in perpendicular fashion an aspheric test surface 238. The light reflected by test surface 238 returns along the original optical path, is superimposed on the reference light beam, reflects off a beam splitting element 256 in lens system 226, and forms interference fringes on a CCD detector 260. By processing these interference fringes by a computer (not shown), the shape error can be measured.
A problem with interferometer 222 is deterioration, in absolute accuracy, due to null element 232. A null element comprising a number of high-precision lenses (e.g., lenses 23425 and 236) a CGH (computer-generated hologram), or the like is ordinarily used as null element 232, and manufacturing errors on the order of 10 nm RMS typically result.
Since interferometer 222 tends to be affected by vibration and air fluctuations due to the separation of reference surface 230 (i.e., rear side of Fizeau plate 228) and test surface 238. Repeatability is also poor, at 0.3 nm RMS. Furthermore, in measuring an aspheric surface, alignment of null element 232 and test surface 238 is critical. Measurement repeatability deteriorates by several nanometers if alignment accuracy is poor.
The present invention relates to an interferometer system for measuring the shape of an aspheric surface of an optical element in an optical system and for measuring the wavefront aberration of such an optical system, particularly in connection with manufacture of a projection optical system suited for use in an exposure apparatus employing soft-X-ray (EUV) exposure light.
The goal of the present invention is to overcome the above-described deficiencies in the prior art so as to permit fast and accurate calibration of a null wavefront corresponding to an aspheric surface accurate to very high dimensional tolerances.
Another goal of the present invention is to manufacture a projection optical system having excellent performance.
Additional goals of the present invention are to provide an aspheric-surface-shape measuring interferometer having good reproducibility, to measure wavefront aberration with high precision and to permit calibration of an aspheric-surface-shape measuring interferometer so as to improve absolute accuracy in precision surface measurements.
Accordingly, a first aspect of the invention is an interferometer capable of measuring a surface shape of a target surface as compared to a reflector standard. The interferometer comprises a light source capable of generating a light beam, and a reference surface arranged downstream of the light source for reflecting the light beam so as to form a reference wavefront. The interferometer further includes a null element arranged downstream of the reference surface for forming a desired null wavefront from the light beam. The null element is arranged such that the null wavefront is incident the target surface so as to form a measurement wavefront and is also incident the reflector standard when the latter is alternately arranged in place of the target surface so as to form a reflector standard wavefront. The interferometer further includes a detector arranged so as to detect interference fringes caused by interference between the measurement wavefront and the reference wavefront. The detection of the interference fringes takes into account the reflector standard wavefront.
A second aspect of the invention is a method of manufacturing a projection optical system capable of projecting a pattern from a reticle onto a photosensitive substrate. The method comprises the steps of first measuring a shape of a test surface of an optical element that is a component of the projection optical system by causing interference between light from the test surface and light from an aspheric reference surface while the test surface and the aspheric reference surface are held integrally and in close proximity to one another. The next step is assembling the optical element in the projection optical system and measuring the wavefront aberration of the projection optical system. The next step is then determining an amount by which the shape of the test surface should be corrected based on the measured wavefront aberration obtained in the step b. Then, the final step is correcting the shape of the test surface based on the amount by which the shape of the test surface should be corrected as determined above.
A third aspect of the invention is an interferometer for measuring wavefront aberration of an optical system having an object plane and an image plane. The interferometer comprises a light source for supplying light of a predetermined wavelength, a first pinhole member capable of forming a first spherical wavefront from the light arranged at one of the object plane and the image plane. The first pinhole member has a plurality of first pinholes arrayed in two dimensions along a surface perpendicular to an optical axis of the optical system. The interferometer further includes a second pinhole member arranged at the opposite one of the object plane and the image plane of the first pinhole member. The second pinhole member has a plurality of second pinholes arrayed at a position corresponding to the imaging position where the plurality of first pinholes is imaged by the optical system. The interferometer also includes a diffraction grating arranged in the optical path between the first and second pinhole members, and a diffracted light plate member that selectively transmits diffracted light of one or more higher predetermined diffraction orders associated with the diffraction grating. The interferometer also includes a detector arranged to detect interference fringes arising from the interference between a second spherical wavefront generated by a zeroeth diffraction order passing through the second pinhole member and the one or more higher predetermined diffraction orders passing through the diffracted light plate member.
A fourth aspect of the invention is an interferometer calibration method for measuring a surface shape of an optical element of an optical system. The method comprises the steps of first, interferometrically measuring the surface shape of the optical element to obtain a surface shape measurement value, then assembling the optical system by including the optical element in the optical system, then measuring a wavefront aberration of the optical system, then separating the wavefront aberration into a component corresponding to positional error of the surface shape and a component corresponding to surface shape error, then correcting the positional error component and calculating the surface shape error component, and then finally correcting the surface shape measurement value using the surface shape error component as previously calculated.